1. Field of the Invention
The present invention relates to an outgoing harmonic level evaluator which computes and evaluates easily a harmonic current generated in a power consumer's facility such as a building and a plant.
2. Description of the Related Art
"Guidelines to the control of harmonics by consumers of high or special high voltage power supply" (issued by the Public Utilities Department, the Agency of Natural Resources and Energy, Japan's Ministry of International Trade and Industry, September 1994) requires that each consumer equipped with a harmonic current sourcing apparatus above an equivalent capacity 50 kVA and receiving a voltage of 6.6 kV at a receiving point control harmonic levels to not exceeding the upper limit of outgoing current per order of the harmonics on a per 1 kW contracted power basis as specified by the guidelines.
FIG. 3 is a skeleton diagram of a receiving and transformation unit in a building or plant regarded as a non-utility electric installation. Referring to FIG. 3, how the amount of generated harmonics is evaluated is shown.
As shown, actual measurements are made to determine whether the level of harmonics exceeds the outgoing current upper limit specified by the above quoted guidelines.
Shown in FIG. 3 are a terminal section 1 of a power cable that leads a three-phase, 50 Hz, 6.6 kV power into a power room in a building or other facility, a disconnector 2, a main circuit-breaker 3, and a potential transformer 4 and a potential current transformer 5 for conducting a voltage and a current at receiving points to respective measuring instruments.
FIG. 3 shows three lighting power systems and two motor driving power systems as feeder lines. Branched off from a load bus line are branch circuit-breakers 13, 23, 33, 43, and 53, to which current transformers 15, 25, 35, 45 and 55 are respectively connected. Transformers 16, 26, 36, 46 and 56 for the lighting power systems and motor driving power systems feed power to loads 17, 27, 37, 47 and 57, respectively.
Also shown in FIG. 3 are a branch circuit-breaker 93 and a current transformer 95 arranged in a phase advancing capacitor block and a vacuum electro-magnetic contactor 113 which engages or disengages a phase advancing capacitor 115 and a serial reactor 114 in or out of a 6.6 kV line. Also shown are vacuum electro-magnetic contactors 123, 133 and 143, serial reactors 124, 134, and 144 and phase advancing capacitors 125, 135, and 145. Using an unshown known automatic power factor regulator, the phase advancing capacitor block is operated to include a number of banks in its bus line required to set the power factor at power receiving points to almost 100%.
The measurements and diagnosis of the amount of generated harmonics in buildings or other facilities are typically made in busiest hours during which a maximum level of harmonics is likely to be generated by unspecified number of loads generating harmonics, for example, 17, 27, 37, 47 and 57 in FIG. 3.
The diagnosis is performed by computing equation (1) based on data obtained from an unshown known harmonic order analyzer that is connected to each of the secondary sides of the current transformers 15, 25, 35, 45 and 55. EQU .vertline.I.sub.n .vertline.=.vertline.I.sub.n1 .vertline.+.vertline.I.sub.n2 .vertline.+ . . . +.vertline.I.sub.n5 .vertline. (1)
where I.sub.n is a total amount of the harmonic currents generated up to an n-th order, and I.sub.n1 through I.sub.n5 are individual amounts of harmonic currents.
Whether an n-th harmonic current in each feeder line is a generated one (outgoing current) or an incoming current is determined by referring to the sign of the n-th harmonic power P.sub.n output from the unshown known harmonic order analyzer. Specifically, if the n-th harmonic power P.sub.n is negative, it is a harmonic generation (outgoing), and if the n-th harmonic power P.sub.n is positive, it is an incoming current.
Alternatively, if the phase angle of the n-th harmonic current I.sub.nm relative to the n-th harmonic voltage V.sub.n is greater than +90.degree. or smaller than -90.degree., respectively, the harmonic power P.sub.n is an outgoing power, and if the phase angle of the n-th harmonic current I.sub.nm relative to the n-th harmonic voltage V.sub.n is within a range of .+-.90.degree., the harmonic power P.sub.n is an incoming power (suffixes m denote feeder lines 1, 2, . . . 5).
Since the phase advancing capacitor blocks 115, 125, 135, 145 are a linear circuit which typically draws in harmonics, the equation (1) for computing the generated harmonic current measured at the secondary side of the current transformer 95 fails to reflect the effect of the phase advancing capacitor blocks 115, 125, 135, and 145 in many cases.
In the prior art evaluator for the outgoing level of harmonic currents, however, harmonic order analysis is performed for each feeder line linked to the respective load, and the amounts of all the generated harmonic currents up to the n-th order are then summed.
In this arrangement, however, the number of analyzers corresponding to the number of feeder lines are needed, or alternatively, when only a single analyzer is available, it has to be repeatedly used for each of the feeder lines, one after another. Furthermore, a determination process of whether the nth harmonic current is an outgoing or incoming one has to be performed on a feeder by feeder basis.